1. Field of the Invention
The present invention relates to an organic light emitting display and a driving method thereof, and more particularly, to an organic light emitting display and a driving method thereof, in which an image is displayed with uniform brightness.
2. Discussion of Related Art
Recently, various flat panel displays have been developed as alternatives to a relatively heavy and bulky cathode ray tube (CRT) display. The flat panel display includes a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), an organic light emitting diode (OLED) display (herein also referred to an organic light emitting display), etc.
Among the flat panel displays, the organic light emitting display can emit light for itself by electron-hole recombination. Such an organic light emitting display has advantages of a relatively fast response time and a relatively low power consumption. Generally, the organic light emitting display employs a transistor provided in each pixel of the display for supplying a current corresponding to a data signal to an organic light emitting diode, thereby allowing the organic light emitting diode to emit light.
FIG. 1 illustrates a conventional organic light emitting display.
Referring to FIG. 1, a pixel 10 of a conventional organic light emitting display emits light corresponding to a data signal supplied to a data line Dm when a scan signal is applied to a scan line Sn.
As shown in FIG. 2, scan signals are applied to first through nth scan lines S through Sn in sequence. Further, data signals are supplied to first through Mth data lines (e.g., the data line Dm), synchronizing with the scan signals.
As shown in FIG. 1, each pixel 10 includes a pixel circuit 12 connected to an organic light emitting diode OLED, the data line Dm and the scan line Sn. The pixel circuit 12 is connected to a first power source ELVDD and applies a current to the organic light emitting diode OLED. The organic light emitting diode OLED includes an anode electrode connected to the pixel circuit 12, and a cathode electrode connected to a second power source ELVSS (or a ground). Here, the organic light emitting diode OLED emits light corresponding to the current supplied from the pixel circuit 12.
In more detail, the pixel circuit 12 includes a second transistor M2 connected between the first power source ELVDD and the organic light emitting diode OLED, a first transistor M1 connected to the data line Dm and the scan line Sn, and a storage capacitor C connected between a gate electrode and a first electrode of the second transistor M2. Here, the first electrode can indicate either of a source electrode or a drain electrode. For example, when the first electrode is selected as the source electrode, the second electrode is selected as the drain electrode. On the other hand, when the first electrode is selected as the drain electrode, the second electrode is selected as the source electrode.
The first transistor M1 includes a gate electrode connected to the scan line Sn, a first electrode connected to the data line Dm, and a second electrode connected to the storage capacitor C. Here, the first transistor M1 is turned on when it receives the scan signal through the scan line S, thereby supplying the data signal from the data line D to the storage capacitor C. At this time, the storage capacitor C is charged with a voltage corresponding to the data signal.
The second transistor M2 includes the gate electrode connected to the storage capacitor C, the first electrode connected to the first power source line ELVDD, and a second electrode connected to the anode electrode of the organic light emitting diode OLED. Here, the second transistor M2 controls the amount of current flowing from the first power source ELVDD to the organic light emitting diode OLED. At this time, the organic light emitting diode OLED emits light with the brightness corresponding to the amount of current supplied from the second transistor M2.
Here, a current flowing in the organic light emitting diode OLED is determined by the following equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                  Vgs                  -                                                          Vth                                                                      )                            2                                =                                    β              2                        ⁢                                          (                                  VDD                  -                  Vdata                  -                                                          Vth                                                                      )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
where, IOLED is a current flowing into the organic light emitting diode OLED, Vgs is a voltage applied between the gate electrode and the first electrode of the second transistor M2, Vth is the threshold voltage of the second transistor M2, Vdata is a voltage corresponding to the data signal, and β is a constant.
Referring to the equation 1, the current flowing into the organic light emitting diode OLED depends on the threshold voltage of the second transistor M2. Thus, each of threshold voltages of second transistors (e.g., the second transistor M2) should be uniform regardless of position of its corresponding pixel (e.g., the pixel 10) in order to display an image with uniform brightness. However, due to possible errors in a fabricating process, each of the threshold voltages of the second transistors (e.g., the second transistor M2) may vary according to the position of its corresponding pixel (e.g., the pixel 10), so that the organic light emitting display may display an image with non-uniform brightness.